The present invention relates to image forming equipment having a revolver type developing device which accommodates a plurality of developing units therein.
A full color copier belongs to a family of image forming equipment of the type electrostatically forming latent images on a photoconductive element, or image carrier, by exposure to color separated light images, developing each latent image by a toner complementary in color to the associated light image, and transferring the resulting toner images to a single sheet one above the other. Also available in the imaging art is multicolor image forming apparatus which forms latent images to be developed in different colors one by one on an image carrier, develops them by developers of different colors, and transfers the resulting toner images to a single sheet. While these types of image forming equipment need a plurality of developing units, the developing units undesirably scale up the equipment when constructed independently of each other and arranged around the image carrier.
A revolver type or rotary developing device is an implementation elaborated to solve the above problem. This type of developing device has a rotatable hollow cylindrical casing facing an image carrier and accommodating a plurality of developing units at predetermined positions therein. The casing is rotated to sequentially locate the developing units at a developing position. As a result, latent images formed on the latent image are each developed by a toner of particular color.
Generally, black-and-white and other monocolor documents are predominant over the others so long as a copier, for example, is operated in an ordinary situation. Hence, a black toner, among the others, is consumed more than the others. The above-stated revolver type developing device cannot accommodate more than a predetermined amount of toner in each developing due to the particular construction. Since this type of device bodily rotates about its own axis, it is difficult to connect a large capacity toner container to the device. Further, while a toner of particular color is consumed more than the others, supplementing the toner from the outside each time is troublesome. In the light of this, Japanese Patent Laid-Open Publication No. 71981/1987, for example, discloses a copier loaded with a monocolor, e.g., black developing device during ordinary copying operation or with a revolver type developing device during color copying. While the copier of this Laid-Open Publication stores toners of two different colors, e.g., red and blue to implement a bicolor or so-called multicolor image, this is also true with a greater number of colors or with a full color developing device storing yellow, magenta and cyan toners.
The spread of copiers and other image forming equipment has given rise to a tendency to entrust ordinary users with, for example, the replacement of units and an image forming device accommodating a plurality of units. Then, the easy and safe replacement of the units and device and the easy handling of substitute units and device are have to be promoted.
Regarding the revolver, each unit may be mounted and dismounted independently of the other units at the front or the top of the equipment body, as taught in, for example, Japanese Patent Laid-Open Publication Nos. 208779/1985 and 127850/1987. Handling the units one by one as mentioned is desirable for the replacement or maintenance of a single unit. However, this kind of approach is not adequate when the units have to be replaced to selectively implement a black-and-white image or a color image.
A plurality of units may be constructed into a single developing device which is bodily removable from the equipment body, as disclosed in Japanese Patent Laid-Open Publication No. 78170/1988 by way of example. This kind of scheme allows a plurality of units to be replaced by a single operation. However, since the units are pulled out at the front of the equipment body, a mechanism for spacing the units apart from the photoconductive element is needed. Moreover, to fully mount or dismount the units, slide rails and guides are indispensable in helping the units slide in the longitudinal direction. Consequently, there are needed a mechanism which promotes smooth mounting and dismounting of the developing device, and a rigid and reliable mechanism which prevents the units from dropping and losing their functions, protects the surroundings and operator from smears due to the toners, and frees the operator from the fear of injury. Such mechanisms are complicated and expensive to increase the cost of the overall equipment.
Further, to pull out the units at the front of the equipment, the front panel of the equipment body has to be formed with an opening greater than the diameter of one unit and is, therefore, mechanically weak. Such a front panel cannot be made thin or has to be provided with a reinforcing member. In addition, the opening has to be provided with a mechanism or a member for positioning the front side of the unit relative to the photoconductive element, further increasing the cost and complicating the mounting and dismounting operation.
The conventional schemes described above have a drawback that the toners are apt to leak from the developing units other than one located at the developing position, contaminating optics, among others, and causing undesirable color mixture to occur. The developer carrier is exposed to the outside via the opening of the casing and, therefore, likely to scratch or otherwise damage the surface of the photoconductive element. Moreover, when the developing device is left at the outside of the equipment or transported, dust, paper dust and other impurities are apt to enter the developing device via the opening to lower image quality and damage the constituent parts.
To eliminate the contamination and color mixture, a cover member or similar screen member having an opening may surround the revolver, as proposed by, for example, Japanese Patent Laid-Open Publication Nos. 172660/1983 and 162271/1985. However, this kind of approach is not satisfactory since the revolver is rotatably supported by the cover member. Specifically, when the revolver is removed from the equipment body and, therefore, brought out of the developing position, the revolver rotates about its own axis relative to the cover member in a pendulum fashion such that the center of gravity thereof is brought to the lowest position. This kind of motion is particularly conspicuous with the unit being used or having been used since the amount of toner consumption in such a unit sharply changes. Assume that such a motion occurs when the user or the serviceman takes out the revolver from the equipment body for a given purpose. Then, the center of gravity of the entire revolver, including the cover member, is shifted to bring the revolver out of balance. As a result, the revolver is likely to fall or contact adjoining devices. This is also true when the revolver is transported, e.g., packages of such revolvers are apt to collapse in the event of shipment.
The prerequisite with the above-described type of revolver is that it be rotated to and held at a predetermined developing position at the time of image formation. To meet this requirement, the revolver is usually brought to a reference position (home position) and then stopped there prior to image formation. However, when the revolver moves in the above-stated manner relative to the cover member after the removal from the equipment body, the position of the revolver relative to the center of rotation becomes arbitrary. Then, when the revolver is inserted into the equipment again, the revolver has to be rotated to the reference position, wasting much time and cost. In addition, the increase in the time and distance for the revolver to reach the reference position aggravates the probability of erroneous detection of the reference position.